Single crystal vane segment and method of manufacture

ABSTRACT

The present invention contemplates a multi-airfoil vane segment produced as a single crystal casting from a rhenium containing directionally solidified alloy. The single crystal casting containing grain boundary strengtheners.

[0001] The present application claims the benefit of the co-pendingProvisional Patent Application Serial No. 60/107,141 entitled SINGLECRYSTAL VANE SEGMENT AND METHOD OF MANUFACTURE, which was filed on Nov.5, 1998, and which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to cast gas turbineengine components and their method of manufacture. More particularly, inone embodiment of the present invention, a multi-airfoil vane segment isproduced as a single crystal casting from a Rhenium containingdirectionally solidified (DS) chemistry alloy. Although the inventionwas developed for gas turbine engine components, certain applicationsmay be outside of this field.

[0003] The performance of a gas turbine engine generally increases withan increase in the operating temperature of a high temperature workingfluid flowing from a combustion chamber. One factor recognized by gasturbine engine designers as limiting the allowable temperature of theworking fluid is the capability of the engine components to not degradewhen exposed to the high temperature working fluid. The airfoils, suchas blades and vanes, within the engine are among the components exposedto significant thermal and kinetic loading during engine operation.

[0004] Many gas turbine engines utilize cast components formed of anickel or cobalt alloy. The components can be cast as a polycrystalline,directionally solidified, or single crystal structure. Generally, themost desirable material properties are associated with the singlecrystal structure. However, the geometry of some components, such as themulti-airfoil vane segmnent, causes difficulty during the castingprocess largely associated with grain or crystal defects. Single crystalalloys are not tolerant to these types of defects and thereforecastings, which exhibit these defects, are generally not suitable forengine use. Thus, the casting yields are lower and consequently the costto manufacture the component increases.

[0005] A directionally solidified component has material propertiesbetween single crystal and polycrystalline and are easier to producethan single crystal components. Directionally solidified components aregenerally defined as multi-crystal structures with columnar grains andare generally cast from a directionally solidified alloy containinggrain boundary strengtheners. The directionally solidified component isbest suited for designs where the stress field is oriented along thecolumnar grains and the stress field transverse to the columnar grain isminimized. However, in a component, such as a multi-airfoil vanesegment, the stress fields are elevated along the airfoils and in atransverse direction associated the inner and outer shrouds which tiethe airfoils together.

[0006] Although the prior techniques can produce single crystalmulti-airfoil vane segments, there remains a need for an improved singlecrystal multi-airfoil vane segment and method of manufacture. Thepresent invention satisfies this and other needs in a novel andunobvious way.

SUMMARY OF THE INVENTION

[0007] One form of the present invention contemplates a productcomprising a cast single crystal structure formed of a directionallysolidified alloy.

[0008] Another form of the present invention contemplates a gas turbineengine component, comprising a single cast single crystal vanesegment.having a plurality of airfoils, the vane segment is formed of adirectionally solidified alloy.

[0009] Yet another form of the present invention contemplates a gasturbine engine component comprising a single cast single crystalshrouded vane formed of a directionally solidified alloy.

[0010] Also, another form of the present invention contemplates a methodfor producing a single crystal article. The method comprising: providinga directionally solidified alloy; melting the directionally solidifiedalloy; pouring the molten directionally solidified alloy into a castingmold; and, solidifying the directionally solidified alloy to produce asingle crystal article.

[0011] One object of the present invention is to provide a singlecrystal multi-airfoil vane segment and method of manufacture.

[0012] Related objects and advantages of the present invention will beapparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is an illustrative view of a gas turbine engine.

[0014]FIG. 2 is a perspective view of a multi-airfoil vane segmentcomprising a portion of the FIG. 1 gas turbine engine.

[0015]FIG. 3 is a Larson-Miller plot comparing three alloys.

[0016]FIG. 4 is an illustrative view of a casting mold for forming avane segment.

[0017]FIG. 5 is an illustrative view of a multi-airfoil vane segmentformed from the casting mold of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018] For the purposes of promoting an understanding of the principlesof the invention, reference will now be made to the embodimentillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the invention is thereby intended, such alterations andfurther modifications in the illustrated device, and such furtherapplications of the principles of the invention as illustrated thereinbeing contemplated as would normally occur to one skilled in the art towhich the invention relates.

[0019] Referring to FIG. 1, there is illustrated a gas turbine engine 20which includes a fan section 21, a compressor section 22, a combustorsection 23, and a turbine section 24 that are integrated together toproduce an aircraft flight propulsion engine. This type of gas turbineengine is generally referred to as a turbo-fan. One alternate form of agas turbine engine includes a compressor, a combustor, and a turbinethat have been integrated together to produce an aircraft flightpropulsion engine without the fan section. The term aircraft is genericand includes helicopters, airplanes, missiles, unmanned space devicesand any other substantially similar devices. It is important to realizethat there are a multitude of ways in which the gas turbine enginecomponents can be linked together. Additional compressors and turbinescould be added with intercoolers connecting between the compressors andreheat combustion chambers could be added between the turbines.

[0020] A gas turbine engine is equally suited to be used for anindustrial application. Historically, there has been widespreadapplication of industrial gas turbine engines, such as pumping sets forgas and oil transmission lines, electricity generation, and navalpropulsion.

[0021] The compressor section 22 includes a rotor 25 having a pluralityof compressor blades 26 coupled thereto. The rotor 25 is affixed to ashaft 27 that is rotatable within the gas turbine engine 20. A pluralityof compressor vanes 28 are positioned within the compressor section 22to direct the fluid flow relative to blades 26. Turbine section 24includes a plurality of turbine blades 30 that are coupled to a rotordisk 31. The rotor disk 31 is affixed to the shaft 27, which isrotatable within the gas turbine engine 20. Energy extracted in theturbine section 24 from the hot gas exiting the combustor section 23 istransmitted through shaft 27 to drive the compressor section 22.Further, a plurality of turbine vanes 32 are positioned within theturbine section 24 to direct the hot gaseous flow stream exiting thecombustor section 23.

[0022] The turbine section 24 provides power to a fan shaft 33, whichdrives the fan section 21. The fan section 21 includes a fan 34 having aplurality of fan blades 35. Air enters the gas turbine engine 20 in thedirection of arrows A and passes through the fan section 21 into thecompressor section 22 and a bypass duct 36. Further details related tothe principles and components of a conventional gas turbine engine willnot be described herein as they are believed known to one of ordinaryskill in the art.

[0023] With reference to FIG. 2, there is illustrated a vane segment 50which forms a portion of a turbine nozzle. A plurality of vane segments50 are conventionally joined together to collectively form the complete360° turbine nozzle. Each of the vane segments 50 include a plurality ofvanes 32 that are coupled to end wall members 51 and 52. The embodimentof vane segment 50, illustrated in FIG. 2, has four vanes coupledthereto, however it is contemplated herein that a vane segment may haveone or more vanes per vane segment and is not limited to a vane segmenthaving four vanes. In a preferred form of the present invention theturbine nozzle includes eleven vane segments having four vanes each.However, a turbine nozzle formed from other quantities of vane segments,and vane segments having other numbers of vanes are contemplated herein.

[0024] Vane 32 has a leading edge 32 a and a trailing edge 32 b and anouter surface extending therebetween. The term spanwise will be usedherein to indicate an orientation between the first end wall member 51and the second end wall member 52. Further, the term streamwise will beused herein to indicate an orientation between the leading edge 32 a andthe trailing edge 32 b. Each vane 50 defines an airfoil with the outersurface 53 extending between the leading edge 32 a and the trailing edge32 b. The leading and trailing edges of the vane extend between a firstend 32 c and a second opposite other end 32 d. The outer surface 53 ofthe vane 50 includes a convex suction side (not illustrated) and aconcave pressure side 55.

[0025] In one embodiment, the gas turbine engine vane 32 is a hollowsingle-cast single crystal structure produced by single crystal castingtechniques utilizing a directionally solidified alloy composition. Inanother embodiment, the gas turbine engine vane is a solid single-castsingle crystal structure produced by single crystal casting techniquesutilizing a directionally solidified alloy composition. Further, thepresent invention contemplates gas turbine engine vanes having internalcooling passageways and apertures for the passage of a cooling media.Cast single crystal casting techniques are believed known to those ofordinary skill in the art. One process for producing a cast singlecrystal structure is set forth in U.S. Pat. No. 5,295,530 to O'Connor,which is incorporated herein by reference.

[0026] In the present invention the material utilized to produce thecast single crystal structure is a directionally solidified alloy, whichoften is referred to as a DS alloy. More preferably, the alloy is asecond-generation directionally solidified superalloy. Second-generationdirectionally solidified superalloys have creep rupture strengthssimilar to first generation single crystal superalloys, such as CMSX-2®and CMSX-3® at up to 1000 degrees centigrade. For example in FIG. 3,there is illustrated a Larson-Miller Plot showing the strength of CM186LC in comparison to CMSX 2/3 and CM247LC. Examples of thesecond-generation superalloys include, but are not intended to belimited herein to: PWA 1426 (a Pratt & Whitney product); René 142 (aGeneral Electric product); and, CM186 LC (a Cannon -Muskegon product).Other directionally solidified alloys are contemplated herein for use inproducing a cast single crystal structure.

[0027] Each of the directionally solidified alloys include grainboundary strengtheners that are designed to increase grain boundarystrength. The alloys PWA 1426, Rene 142 and CM186 LC each include boron,carbon, hafnium, and zirconium as their grain boundary strengtheners.Other directionally solidified alloys containing grain boundarystrengtheners are contemplated herein. A grain boundary is generallydefined as a region in the cast component of non-oriented structurehaving a width of only a few atomic diameters which serves toaccommodate the crystallographic orientation difference or mismatchbetween adjacent grains. It will be appreciated by those skilled in theart that neither low angle grain boundaries nor high angle grainboundaries will be present in a theoretical “single crystal”. However,it will be further appreciated that although there may be one or moregrain boundaries present in commercial single crystal structures, theyare still characterized as a single crystal structure. Further,manufacturing processes more tolerant of these crystal anomalies areinherently less expensive.

[0028] The nominal chemical composition for the Rhenium containingalloys PWA 1426, Rene 142 and CM186 LC are disclosed in Table I. TABLE INOMINAL COMPOSITION, WEIGHT % Density Alloy Cr Co Mo W Ta Re Al Ti Hf CB Zr Ni (kg/dm) PWA 6.5 12 2 6 4 3 6.0 — 1.5 .10 .015 .03 BAL 8.6 1426René 6.8 12 2 5 6 3 6.2 — 1.5 .12 .015 .02 BAL 8.6 142 CM 186 6.0 9 .5 83 3 5.7 .7 1.4 .07 .015 .005 BAL 8.70 LC

[0029] With reference to FIG. 4, there is illustrated a casting mold 200with a molten metal receiving cavity for receiving molten metal thereinand forming the multi-airfoil vane segment. Referring to FIG. 5, thereis illustrated the multi-airfoil vane segment 50 and metallic starterseed 62 with the walls of a casting mold 200 removed to aid the reader.A portion of the metallic starter seed 62 extends into the molten metalreceiving cavity of the mold. The molten directionally solidified alloycontacts the starter seed 62 and causes the partial melt back thereof.In a preferred form of the process for producing the cast multi-airfoilvane segment the starter seed 62 is not in contact with a chill 65. Morepreferably an insulator 90 is disposed between the starter seed 62 andthe chill 65. The insulator 90 functions to thermally insulate thestarter seed 62 from the cooling chill 65 and thus promote melting of aportion of the starter seed.

[0030] The directionally solidified alloy is solidified by a thermalgradient moving vertically through the casting mold. More particularly,the directionally solidified alloy is solidified epitaxially from theunmelted portion of the starter seed 62 to form the single crystalproduct. In one form, the thermal gradient for solidifying thedirectionally solidified alloy is produced by a combination of moldheating and mold cooling. One system for effectuating the thermalgradient in the mold comprises a mold heater, a mold cooling cone, achill and the withdrawal of the structure being cast. Further detailsrelated to the growing of single crystal alloy structures are believedknown to those of ordinary skill in the art and therefore have not beenprovided. The cast single crystal alloy product has been described interms of a vane segment, however other cast single crystal productconfigurations formed of a directionally solidified alloy, such asblades seals, shrouds, blade tracks, nozzle liners and other componentssubjected to high temperature and stress are contemplated herein.

[0031] In one form of the present invention the starter seed 62 isformed and/or oriented such that the seeds <001> (primary orientation)crystal direction is substantially parallel with a tangent A, and theseeds <010> (secondary orientation) crystal direction is substantiallyparallel with the average airfoil stacking axis B. The average airfoilstacking axis B is generally defined by the average of each airfoilstacking axis B₁, B₂, B₃, and B₄. The illustration of FIG. 5 is notintended herein to limit the solidification direction to that shown inthe drawings. In an alternative embodiment the solidification directionis substantially parallel to the average airfoil stacking axis B.Further, other solidification directions are contemplated herein. Thepresent invention is not limited to the use of a starter seed to impartthe crystallographic structure to the crystal being grown Singlecrystals can be grown by techniques generally known to one of ordinaryskill in the art, such as utilizing thermal nucleation and the selectionof a grain for continued growth with a pigtail sorting structure.

[0032] In one form the cast single crystal vane segment can be usedwithout the long homogenization heat treat cycles commonly used tomaximize properties of cast single crystal articles. In another form ofthe present invention, which is well suited for articles such as gasturbine blades, the article can be used in a fully heat treatedcondition. The fully heat treated article maximizes stress rupture andminimizes the formation of deleterious topologically close packed (TCP)phases such as sigma upon the long term exposure of the article to hightemperature and stress. The long term exposure will be greater than onethousand hours.

[0033] While the invention has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character, it being understoodthat only the preferred embodiment has been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. A product comprising a cast single crystalstructure formed of a directionally solidified alloy.
 2. The product ofclaim 1, wherein said directionally solidified alloy is selected fromthe group consisting of PWA 1426, René 142 and CM186 LC.
 3. The productof claim 1, wherein said directionally solidified alloy includesRhenium.
 4. The product of claim 3, wherein said directionallysolidified alloy includes about 3 weight percent Rhenium.
 5. Thecomponent of claim 1, wherein said alloy consisting essentially of, inpercentages by weight, 0.07 C, 6 Cr, 9 Co, 0.5 Mo, 8 W, 3 Ta, 3 Re, 5.7Al, 0.7 Ti, 0.015B, 0.005 Zr, 1.4 Hf, the balance being nickel andincidental impurities.
 6. The component of claim 1, wherein said alloyconsisting essentially of, in percentages by weight, 6.8 Cr, 12 Co, 2Mo, 5 W, 6 Ta, 3 Re, 6.2 Al, 1.5 Hf, 0.12 C, 0.015 B, 0.02 Zr, thebalance being nickel and incidental impurities.
 7. The component ofclaim 1, wherein said alloy consisting essentially of, in percentages byweight, 6.5 Cr, 12 Co, 2 Mo, 6 W, 4 Ta, 3 Re, 1.5 Hf, 0.10 C, 0.015 B,0.03 Zr, 6.0 Al, the balance being nickel and incidental impurities. 8.A gas turbine engine component, comprising a single cast single crystalvane segment having a plurality of airfoils, vane segment formed of adirectionally solidified alloy.
 9. The component of claim 8, whereinsaid vane segment has a first member and a second member spacedtherefrom, and wherein each of said plurality of airfoils are integrallycast with and extend between said first and second members.
 10. Thecomponent of claim 9, wherein said alloy consisting essentially of, inpercentages by weight, 0.07 C, 6 Cr, 9 Co, 0.5 Mo, 8 W, 3 Ta, 3 Re, 5.7Al, 0.7 Ti, 0.015 B, 0.005 Zr, 1.4 Hf, the balance being nickel andincidental impurities.
 11. The component of claim 9, wherein said alloyconsisting essentially of, in percentages by weight, 6.8 Cr, 12 Co, 2Mo, 5 W, 6 Ta, 3 Re, 6.2 Al, 1.5 Hf, 0.12 C, 0.015 B, 0.02 Zr, thebalance being nickel and incidental impurities.
 12. The component ofclaim 9, wherein said alloy consisting essentially of, in percentages byweight, 6.5 Cr, 12 Co, 2 Mo, 6 W 4 Ta, 3 Re, 1.5 Hf, 0.10 C, 0.015 B,0.03 Zr, 6.0 Al, the balance being nickel and incidental impurities. 13.The component of claim 9, wherein at least one of said plurality ofairfoils has an internal cooling passageway for the passage of a coolingmedia.
 14. The component of claim 8, wherein said directionallysolidified alloy includes a grain boundary strengthener.
 15. A gasturbine engine component comprising a single cast single crystalshrouded vane formed of a directionally solidified alloy.
 16. Thecomponent of claim 15, wherein said directionally solidified alloyincludes at least one grain boundary strengthener.
 17. The component ofclaim 16, wherein said at least one grain boundary strengthener includesboron, carbon, hafnium and zirconium.
 18. The component of claim 15,wherein said directionally solidified alloy includes about 3 weightpercent Rhenium.
 19. A method for producing a single crystal article,comprising: providing a directionally solidified alloy; melting thedirectionally solidified alloy; pouring the molten directionallysolidified alloy into a casting mold; and solidifying the directionallysolidified alloy to produce a single crystal article.
 20. The method ofclaim 19, which further includes moving a thermal gradient through thecasting mold.
 21. The method of claim 19, which further includesproviding a metallic starter seed, and wherein a portion of the metallicstarter seed is positioned within the casting mold.
 22. The method ofclaim 21, which further includes partially melting back the starterseed.
 23. The method of claim 22, wherein in said solidifying thedirectionally solidified alloy is solidified epitaxially from anunmelted portion of the starter seed.
 24. The method of claim 21, whichfurther includes providing a chill to withdraw energy through saidstarter seed.
 25. The method of claim 24, which further includesthermally insulating the starter seed from the chill to promotepartially melting back the starter seed.
 26. The method of claim 25,wherein said thermally insulating including placing an insulator betweenthe chill and the starter seed.
 27. The method of claim 21, whichfurther includes aligning the starter seed such that its <001> crystaldirection is substantially parallel with a tangent to the vane segment,and the starter seeds <010> crystal direction is substantially parallelwith an average airfoil stacking axis.